1. Field of the Invention
The invention relates to a torque sensor having a first signal generator, whose output signal changes as a function of a torque, a second signal generator, whose output signal changes in the opposite direction as a function of the torque, and an evaluation device which has a summing device which is connected to both signal generators. In addition, the invention relates to a method of producing a torque-dependent signal from output signals of two signal generators with opposed signal waveforms, in which the sum of the output signals is formed.
2. Description of the Related Art
A torque sensor and a method of producing a torque-dependent signal of the above-mentioned type are disclosed by EP 0 555 987 B1. The torque sensor disclosed is used in a power-assisted vehicle steering system. It is arranged between a steering wheel and a steering gearbox. The steered wheels are acted on by a motor which is activated in such a way that the torque determined by the torque sensor becomes as small as possible. The signal generators operate optically. A light source radiates through an aperture arrangement onto a photoelectric region which is arranged at one end of a torsion rod. The aperture arrangement is connected to the other end of the torsion rod. When a torque is applied, the torsion rod twists somewhat. Consequently, the aperture is displaced with respect to the photoelectric pickups, so that the first pickup receives a greater amount of light and the second pickup receives a smaller amount of light than in the neutral position. Consequently, the output signals change. The difference between the output signals is divided by the sum of the output signals, in order to achieve the situation in which the torque-dependent signal becomes independent of changes in the light intensity or of age-induced changes in the components.
A different torque sensor is disclosed by EP 0 765 795 A2. Here use is likewise made of two signal generators, whose output signals vary in opposite directions as a function of position. The signal generators are fed by a supply voltage. The supply voltage is then controlled in such a way that the sum of the two output signals is constant. This torque sensor is also used for operating a power-assisted steering system.
The present invention provides a system and method for obtaining a torque signal in a simple way.
In a torque sensor of the type mentioned at the beginning, this object is achieved by the first signal generator being connected to an integrator whose output is connected to a first input of a comparator whose second input is connected to the summing device, the output of the comparator being connected to a reset input of the integrator and to a signal output from the torque sensor.
In this configuration, both a controller which has to keep a supply voltage to a constant value, and a divider which is used for normalization, are dispensed with. Nevertheless, a torque-dependent signal is available at the output of the comparator. In this case, its frequency is torque-dependent. The integrator integrates the integrates the output signal of the first signal generator until this is equal to the sum of the two output signals. As long as the condition still has not been satisfied, the output of the comparator has a first state, for example 5 V or xe2x80x9chighxe2x80x9d. As soon as the output voltage has reached the voltage sum of the output signals, the output of the comparator goes to xe2x80x9clowxe2x80x9d or to 0 V. Accordingly, one pulse is produced at each changeover in a specific direction. At this changeover, the comparator then resets the integrator, so that the integration can begin from the start. It is obvious that the frequency depends on the magnitude of the output signal of the first signal generator. In the case of a greater signal, the frequency is higher, since the integration time is shorter, and in the case of a smaller signal the frequency is correspondingly lower. Since the sum of the output signals is used as a criterion for the changing over of the comparator, far-reaching independence from similar changes in these signals is achieved, these changes being caused by components, for example.
In a preferred configuration, the output of the comparator is connected to a pulse generator. At the changeover, the pulse generator in each case produces a pulse of predetermined length. Pulses of this type can subsequently be processed better.
The summing device is preferably constructed as a summing amplifier. It therefore not only sums the output signals of the two signal generators but further amplifies this sum. In this way, longer integration times are achieved and, as a result, somewhat lower frequencies at the output of the comparator. Furthermore, a further influencing variable for configuring the frequency is obtained via the selection of the gain.
In this case, it is particularly preferable for the gain of the summing amplifier and the time constant of the integrator to be selected such that, given equality between the output signals, a signal at a frequency of at least 3 kHz is set at the output of the comparator. If the output signals are equal, there is no torque. A frequency of at least 3 kHz may be processed easily. It still allows sufficient latitude, upward and downward, to be able to evaluate torque changes upward and downward.
The signal output is preferably connected to a wire-free signal transmission path. A wire-free signal transmission path of this type may operate optically or with radio signals, for example. Since the output signal contains the information in the frequency, such a transmission is generally possible without problems. The wire-free signal transmission path has the advantage that it can be used in rotating components as well. This case occurs primarily in steering systems.
The input of the integrator is preferably connected to a second summing device, whose first input is connected to the first signal generator and whose second input is connected to the output of the first summing device. With this configuration, it is possible to implement an offset shift of the frequency, that is to say the midfrequency which is produced without any torque can be raised, so that, even at extreme values of the torque, the output frequency is sufficiently high to permit short measuring cycles.
A proportionality element is preferably arranged between the two summing devices. This proportionality element does not have to be constructed as a discrete component. It can also be formed by a feedback means with which the gain of the summing device is changed. The proportionality factor of the proportionality element is included directly in the offset by which the frequency is shifted. By means of an appropriate selection of this proportionality factor, the desired mid-frequency can therefore be fixed relatively accurately.
A second proportionality element is preferably arranged between the first summing device and the comparator. The same applies here with regard to the construction as for the first proportionality element, that is to say it can also be formed by appropriate feedback of an amplifier. Using this second proportionality element, the slope of the frequency against torque may be adjusted. It is thus possible to influence the sensitivity of the sensor.
In a particularly preferred configuration, provision is made for the first input of the second summing device to be connected to the output of a difference-forming unit whose two inputs are connected to the two signal generators, and for the second input of the second summing device to be connected to the input of the comparator via a proportionality element, the input signal of the comparator being formed from the sum signal amplified by a gain, and the proportionality factor of the proportionality element being less than 1, but greater than the reciprocal of the gain. In this configuration, the frequency slope and the frequency offset can be set independently of each other.
The second signal generator is preferably connected to a second integrator having a second comparator downstream, a monitoring device being provided which checks the sum of the frequencies at the outputs of the comparators for constancy. This configuration is particularly advantageous when used in safety-relevant applications. The functioning of the circuit is capable of diagnosis. Since the output signals from the signal generators run in complementary fashion to each other, the sum of the frequencies at the output of the comparators remains constant and equal to twice the frequency of the zero-torque signal. Relatively small fluctuations are of course permitted. If, however, the frequency sum leaves a predetermined corridor, this is a relatively sure sign of the occurrence of a fault.
In the case of a method of the type mentioned at the beginning, the object is achieved by an output signal being integrated until the integrated signal corresponds to the sum of the output signals.
As explained above in conjunction with the sensor, in this configuration a signal is obtained whose frequency is proportional to the torque. By selecting the sum of the output signals as a reference variable for the end of the integration, a large degree of independence is obtained from synchronous changes in the individual output signals.